Ongoing refinement of immunosuppression regimens has substantially reduced the incidence of acute rejection after solid organ transplantation. However, long-term outcomes have stagnated partly due to morbidity and mortality associated with immunosuppression. The traditional approach to immunosuppression has emphasized non-specific suppression of T cell responses.
The more recent elucidation of T regulatory cells (Tregs) and their importance in regulating immune responses has encouraged the reconfiguration of immunosuppression regimens to favor Treg development and function with the ultimate goal of inducing graft tolerance (Waldmann et al., J. Clin Immunol, 28:716-725, 2008; Kang et al., Am J Transplant, 7:1457-1463, 2007; Walsh et al., J Clin Invest, 114:1398-1403, 2004; Yeung et al., Transplant Proc, 41:S21-26, 2009; Sanchez-Fueyo et al., J Immunol, 176:329-334, 2006; Sagoo et al., Curr Opin Organ Transplant, 13:645-653, 2008; and Long et al., Transplantation, 88:1050-1056, 2009). Multiple preclinical models have shown that adoptive transfer of Tregs can mitigate graft rejection and, in combination with “Treg-supportive” immunsuppression regimens, can induce long-term tolerance (Kang et al., Am J Transplant, 7:1457-1463, 2007; Riley et al., Immunity, 30:656-665, 2009; Issa et al., Expert Rev Clin Immunol, 6:155-169, 2010; and Nadig et al., Nat Med, 16:809-813, 2010). Treg-supportive” immunsuppression regimens have included the initial de-bulking of donor-reactive T cells. Rabbit anti-thymocyte globulin (rATG), a commonly used T-cell depleting agent in transplantation, appears to spare Tregs (Sewgobind et al., Nephrol Dial Transplant, 24:1635-1644, 2009), thereby increasing Treg:T conventional cell (Tconv) ratio. Additionally, sirolimus (SRL) suppresses effector T cells while fostering Treg development (Demirkiran et al., Transplantation, 85:783-789, 2008; and Demirkiran et al., Transplantation, 87:1062-1068, 2009).
Most protocols typically expand all Tregs nondiscriminately to produce cells referred to as polyclonal Tregs (polyTregs). However, alloantigen-specific Tregs (alloTregs) are more effective and safer than non-specific Tregs in transplant settings because they provide specific rather than generic immunosuppression (Golshayan et al., Blood, 109:827-835, 2007; and Raimondi et al., J Immunol, 184:624-636, 2010). In particular, donor-reactive Tregs have the potential to induce tolerance to the transplanted organ without impeding conventional immune responses. Thus what is needed in the art are robust methods for expansion of alloTregs for use in promoting transplant tolerance and for treating graft versus host disease.